Tunnel pick-off vibrating rate sensor

ABSTRACT

A rate sensor has a tine cut from a silicon substrate and resiliently mounted by two integral flexure beams. An electrostatic actuator drives the tine to vibrate in its plane. The tine supports two tunnel pick-offs in the form of spikes projecting from the tine, the tips of the spikes being positioned below a pick-off plate. In operation, the plate is moved down until it is closely spaced from the spikes and a tunneling current is produced. One spike is positioned below a recess so that there is a sharp fall in current when the spike passes beneath the recess, this signal being used to indicate the amplitude and frequency of vibration. The tunneling current output from the other spike indicates the separation from the plate. Rotation of the sensor about an axis y in the plane of vibration of the tine causes displacement of the tine at right angles, which is sensed by the tunnel pick-off.

FIELD OF THE INVENTION

This invention relates to rate sensors of the kind including aresiliently-mounted element, an actuator that vibrates the element in afirst plane, and a displacement sensor responsive to displacement of thevibrating element in a sensing direction normal to the first planecaused by rotation of the sensor about an axis in the plane and at rightangles to the sensing direction.

BACKGROUND OF THE INVENTION

Vibrating element rate sensors, such as tuning fork gyros, have anelement driven to vibrate in one plane. When the sensor is subject torotation about an axis parallel to the vibration plane, a force isproduced on the vibrating element orthogonal to the axis of rotation andthe plane of vibration. This tends to cause deflection of the element,which is sensed by a suitable sensor, such as a capacitive pick-off.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an improved ratesensor.

According to one aspect of the present invention there is provided arate sensor of the above-specified kind, characterized in that thedisplacement sensor includes a tunnel pick-off having a first partlocated on the vibrating element and a second part separate from thevibrating element, one part including a spike and the other partincluding a surface closely spaced from the tip of the spike.

The spike is preferably mounted on the vibrating element, the surfacebeing on the second part. The tunnel pick-off may include two spikes,the surface having a recess located above one of the spikes, the outputof the spike located beneath the recess being utilized to provide anoutput representative of vibration of the element. The sensor mayinclude an actuator for maintaining constant average separation betweenthe first part and the second part of the pick-off. The sensor mayinclude an actuator so that the first and second parts can be displacedrelative to one another from a position prior to use in which each spikeis protected. The surface may have a recess positioned above each spikeprior to use. The vibrating element is preferably a plate machined froma silicon substrate, the substrate supporting the second part of thetunnel pick-off. The actuator that vibrates the element in the firstplane is preferably an electrostatic actuator. The vibrating element mayhave two flexure arms, the vibrating element being tuned by removingmaterial from a surface of the arms.

According to another aspect of the present invention there is provided atwo-axis inertial rate sensor system including four pairs of ratesensors according to the above one aspect of the invention,characterized in that the rate sensors in each pair are mirror images ofone another.

Other aspects of the present invention will become apparent from thefollowing description, by way of example, of a two-axis inertial ratesensor system, including sensors according to the present invention,with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view of the sensor;

FIGS. 2A and 2B are sectional side elevation views, to an enlargedscale, of the pick-off assembly along the line II--II in FIG. 1, showingthe assembly at a rest and operational state respectively;

FIG. 3 is a sectional side elevation, to an enlarged scale, showing apart of the vibrating element along line III--III of FIG. 1;

FIG. 4 is a plan view of the system; and

FIG. 5 is a plan view of a sensor with alternative actuators.

DETAILED DESCRIPTION OF THE DRAWINGS

With reference first to FIGS. 1 to 3, the sensor includes a siliconwafer substrate 1 of rectangular shape having a tine 2. The tine 2 isformed integrally from the wafer by using microengineering techniques,such as photolithography or micromachining, to cut an aperture 3 ofinverted U shape. The tine 2 is located in the lower half of the wafer1, the aperture 3 extending around three sides of the tine. A secondaperture 4 is cut laterally across the middle of the bottom of the tine2 to leave two vertical flexure beams 5 and 6 providing the sole supportfor the tine in the wafer 1. The beams 5 and 6 are of rectangularsection having their shorter sides parallel to the plane of the wafer,as shown in FIG. 3. Between the beams 5 and 6, the tine has sevenparallel teeth 7 extending vertically down and forming one half of anelectrostatic comb actuator 8. The other half of the actuator 8 isprovided by an assembly 9 mounted on the wafer 1 itself below the tine2, which has six teeth 10 projecting upwardly between the teeth 7. Theactuator 8 is connected to a drive unit 71, which applies an oscillatingvoltage between the two sets of teeth 7 and 10 so that the tine 2 isdriven to vibrate in the direction "x" in the plane of the tine, atright angles to the length of the beams 5 and 6. More particularly, theteeth 7 are energized so that each tooth in the same comb has theopposite polarity from the adjacent tooth (+, -, +, - and so on). Also,the teeth 8 are similarly energized so that they each have oppositepolarities from adjacent teeth in the same comb. In this way, thepositively-charged teeth 7 are attracted to the negatively-charged teeth8, and the negatively-charged teeth 7 are attracted to those teeth 8that are positively charged, causing movement of the tine 2 in onedirection, at right angles to the length of the teeth. Reversing thepolarity on one set of teeth causes the tine 2 to move in the oppositedirection.

The upper end of the wafer 1 supports a tunnel pick-off assembly 20 andan out-of-plane actuator assembly 30. A part of the tunnel pick-offassembly 20 is provided by two tunnel nanotips or spikes 21 and 21'projecting upwardly at right angles to the surface of the tine 2,side-by-side close to its upper edge. The spikes 21 and 21' are ofconical shape and typically have a tip radius of about 5 nm. A tunnelpick-off is defined here as being one having a sharp discontinuity orspike that strips electrons from an adjacent surface when brought intoclose proximity, to produce a current that is proportional to theseparation between the surface and the discontinuity. More than twospikes can be provided with only the outputs from the highest spikesbeing utilized. The spikes 21 and 21' may be formed by any conventionaltechnique, such as by deposition through a stand-off mask or bylaser-induced chemical vapor deposition (LICVD). The spikes may be of adiamond material. The other part of the pick-off assembly 20 is providedby a plate 22 extending parallel to the wafer 1 and closely spaced aboveit. The plate 22 is of rectangular shape, overlapping the upper regionof the tine 2 on which the spikes 21 and 21' are located. The plate 22has a conductive surface 27 on its underside and two small circularholes or similar recesses 51 and 51' through its thickness, which arepositioned above the tunnel spikes 21 and 21' respectively when theplate 22 is in its rest position before use, as shown in FIG. 2a Thepick-off plate 22 also has a hole or recess 52, of triangular shape,located a short distance below the left-hand circular hole 51. Thecircular holes 51 and 51' may be used as the stand-off masks throughwhich the spikes 21 and 21' are deposited. The spikes 21 and 21' and theconductive underside 27 of the pick-off plate 22 are electricallyconnected to an electronics unit 72 to 75 functioning as a tunnelpick-off unit by which a voltage can be applied between the spikes andthe plate.

Two resilient suspension arms 23 and 24 project laterally on either sideof the plate 22 and are terminated by short pillars 25 and 26respectively attached to the upper surface of the wafer 1. The naturalfrequency of the pick-off plate suspension is selected to be higher thanthat of the tine 2. The suspension arms 23 and 24 hold the plate 22above the tine 2 but allow it to be deflected up or down normal to theplane of the wafer along an axis "z", by the action of the out-of-planeactuator assembly 30.

The out-of-plane actuator assembly 30 is of an electrostatic kind andcomprises a lower metal electrode 31, mounted on the upper surface ofthe wafer 1 below the plate 22, and an upper electrode 32 mounted on theunderside of the plate, directly above the lower electrode. Theelectrodes 31 and 32 are connected to the electronics unit 72 to 75,which acts as an out-of-plane actuator drive unit. The suspension arms23 and 24 also allow the plate 22 to be displaced along a line parallelto the plane of the wafer 1 and at right angles to the arms, shown asthe "y" axis. Displacement of the plate 22 along the y axis, parallel tothe plane of the wafer 1, is effected by means of an in-planeelectrostatic actuator assembly 40.

The in-plane actuator assembly 40 comprises a first part formed by apart of the plate 22 itself, in the form of an actuator arm 41projecting upwardly along the y axis, the arm having four fingers 42projecting outwardly from each side. The other part of the in-planeactuator assembly 40 comprises two sub-assemblies 43 and 44, one on eachside of the arm 41. Each sub-assembly 43 and 44 has five fingers 45extending inwardly towards one another, parallel to the x axis, whichare interlaced with the fingers 42 on the arm 41. The fingers 42 and 45are electrically connected to the electronics unit 72 to 75, which alsoserves as an in-plane actuator drive unit. The electronics unit 72 to 75can apply a voltage between the fingers 42 and 45 to cause the plate 22to be displaced either up or down along the y axis, at right angles tothe length of the fingers, in the same way as with the tine actuator 8.

Before the sensor is initially started up, the pick-off plate 22 isspaced above the tip of the spikes 21 and 21' by a short distance andthe spikes are aligned with the holes 51 and 51', as shown in FIG. 2A.In this way, the spikes 21 and 21' are protected, because, even if theplate 22 were deflected down towards the tine 2, it would not contactthe spikes. On start up, a voltage is applied to the in-plane actuatorassembly 40 sufficient to pull the pick-off plate 22 upwardly along they axis, away from the tine 2, by a distance sufficient to move theprotective holes 51 and 51' away from the spikes 21 and 21' and toposition the triangular hole 52 directly above the left-hand spike 21,as shown in FIG. 2B. In this position, the right-hand spike 21' islocated below a plane surface provided by the underside 27 of the plate22. A drive voltage is then applied to the tine actuator assembly 8 sothat the tine 2 is driven to vibrate in the plane of the wafer 1 alongthe axis x. The actuator drive unit 71 is self tuning so that the tine 2is driven at its resonant frequency at some nominal amplitude initially.A ramp voltage is then applied by the electronics unit 72 to 75 to theout-of-plane actuator assembly 30, so that the pick-off plate 22 isgradually moved down parallel to the z axis, towards the upper surfaceof the oscillating tine 2. As the lower surface of the pick-off plate 22comes closer to the tip of the right-hand spike 21', current begins toflow because of the tunnelling effect. The magnitude I of the current isgiven by the expression:

    I∝Ve.sup.-α√φs

Where V=bias voltage

φ=tunnel barrier height

    α=1.025 (Å.sup.-1 eV.sup.-1/2)

s=separation

Sensitivity of the sensor is about one order of magnitude for each Åchange in separation.

As the tine 2 moves backwards and forwards, the tip of the spike 21'will move across a small arc of the underside of the plate 22 and thecurrent will vary because of imperfections in the surface, even down tothe atomic scale. The electronics unit 72 to 75 acts as a pick-off unitto monitor the mean current produced as the plate 22 is brought closerto the tip of the spike 21' and, when this reaches a predeterminedvalue, corresponding to a predetermined separation, the ramp voltage isterminated and a servo voltage is applied so that this averageseparation is maintained. The separation between the tip of the spikesand the surface 27 of the plate 22 is of the same order as the tipradius, that is, about 5 nm. The left-hand spike 21 produces a currentoutput in the same way but, because this spike is located below thetriangular hole 52, there will be a sharp drop in current when the spikepasses under the hole. The electronics unit 72 to 75 monitors thisoutput to derive a measure of the amplitude and frequency of oscillationof the tine 2--this is used to servo control the magnitude of the signalapplied by the in-plane actuator assembly 30.

When there is no rate input to the sensor, the tine 2 vibrates in theplane of the wafer 1 at a constant servo-controlled amplitude. Thepick-off output current corresponds to the average servo-controlled gapbetween the tine 2 and the pick-off plate 22.

Immediately an input rate is applied about the y axis, parallel to theplane of the wafer 1 and at right angles to the linear vibration of thetine 2, the tine will experience an oscillating Coriolis force tendingto cause it to vibrate in a direction normal to the plane of the wafer,along the z axis. The frequency of this Coriolis force will be the sameas the drive frequency of the tine. These minute oscillations are sensedby the pick-off assembly 20 and appear as a modulation of the meantunnel current. The amplitude of the modulation corresponds to themagnitude of the input rate. The phase of the modulation, compared withthe phase of the drive voltage or the amplitude output of the pick-off20, indicates the sense of the input rate, positive or negative.

Alternatively, it may be possible to have a fully closed loop systemwhere the tunnel gap is servoed to maintain a constant tunnel current;the out-of-plane actuator servo current then becomes a measure of inputrate.

Because the tunnel pickoff is sensitive to very small distances, itsoutput will include noise caused by the surface discontinuities on theunderside of the pick-off plate 22. However, the tunnel pick-off willtrace the same path across the plate 22 for each oscillation, so thenoise is cyclic and repeated, enabling it to be removed by digitalsignal processing or active filtering techniques.

The Coriolis force is directly proportional to the linear velocity ofthe tines and this is proportional to the amplitude of vibration, so thesensitivity of the sensor can be readily altered by adjusting theamplitude of vibration of the tine 2 to accommodate different inputrates. The high sensitivity of the tunnel pick-off means that only verysmall vibration in the sensing plane is needed and that the sensingamplitude can be considerably smaller than the drive amplitude, therebyminimizing non-linearities of response. It also means that the driveamplitudes can be relatively small, minimizing hysteresis and couplinglosses to the surrounding structure.

The inertial rate system shown in FIG. 4 comprises four pairs of sensorsindicated by the labels Y1 to Y4 and X1 to X4. Each sensor in a pair isa mirror image of the other sensor in the pair. The eight sensors aremounted on a square substrate 70, about 2 cm square, together with thefive associated electronics units 71 to 75. The sensors and electronicsunits are preferably formed directly in the substrate, with the wafer 1being a part of a silicon substrate, although they could be formedseparately and subsequently mounted on the substrate. Each pair ofsensors Y1 and Y2, Y3 and Y4, X1 and X2, and X3 and X4 is mountedcentrally along the sides of substrate 70 with the unit 71 locatedcentrally and the other units 72 to 75 located in opposite comers. Thesubstrate could have additional slots and apertures to provide aflexible mounting of the sensors and thereby mechanically isolate themfrom the outside world. The central unit 71 contains electronics fordriving the tines 2 of all eight sensors, the other four units 72 to 75contain electronics for driving the in-plane actuator 40, theout-of-plane actuator 30 and for processing the outputs of the pick-offassemblies 20 and the out-of-plane actuators 30 of adjacent pairs ofsensors. The sensors are arranged so that both the x and y axis inputshave four inertially-balanced sensors with four tines vibrating inantiphase so that the tunnel pick-offs can provide a differential outputsignal with a good common mode rejection. This allows independent ratedetection about two orthogonal axes.

For maximum sensitivity, the resonant frequency of all eight tines inthe system should be identical both in the plane of driven vibration andat right angles to this, in the sensing plane. To achieve this, thetines are initially machined to give them a natural frequency slightlyhigher in the sensing plane than in the driven plane. The tines are thenfrequency trimmed by ablating a small amount of material from the upperor lower surface, or both surfaces, of each flexure beam 5 and 6, closeto a point P of maximum stress. This removal of material may be carriedout by a focussed electron beam or laser. As shown in FIG. 3, theflexure beams 5 and 6 have a neutral axis N₁ for in-plane vibration thatextends vertically out of the plane centrally across the width of thebeam, and a neutral axis N₂ for out-of-plane vibration that extendshorizontally centrally across the thickness of the beam. Removal of thematerial from the upper or lower surface of the beams 5 and 6 will havea greater effect on the resonant frequency in the driven (in-plane)direction than on the frequency in the sensing (out-of-plane) direction,because the material removed is very close to the neutral axis in thedriven plane but is at some distance from the neutral axis in thesensing plane. Tuning is achieved by simultaneously exciting the tines 2in both directions and monitoring their frequencies via their respectivetunnel pick-off assemblies 20. The tines 2 are excited in theout-of-plane direction by applying an intermittent drive signal to theunderside of the pick-off plate 22, with the out-of-plane actuator 30being used to cancel out the reaction force and prevent the pick-offplate going into resonance. The beams are automatically trimmed untilthe two resonant frequencies become equal.

The electrostatic actuators 8 and 40 described above have two combs withinterdigitated fingers or teeth, one comb moving at right angles to thelength of the fingers or teeth. FIG. 5 shows a sensor with a tineactuator 8' and an in-plane actuator 40' where movement is effectedparallel to the length of the fingers or teeth. The tine actuator 8' hasa central structure 80 fixed on the wafer 1', with a set of five teeth81 projecting to the left and five teeth 82 projecting to the right.These two sets of teeth 81 and 82 are interdigitated with respectivepairs of teeth 83 and 84 on the tine 2', which extend at right angles tothe length of the tine. Oscillation of the tine 2' is produced byapplying a voltage of the same polarity to all the tine teeth 83 and 84,and by applying a voltage of one polarity to the teeth 81, on one sideof the fixed structure 80, and of the opposite polarity to the teeth 82on the opposite side of the fixed structure. For example, if the teeth83 and 84 were to have a positive charge, the teeth 81 a positive chargeand the teeth 82 a negative charge, the tine 2' would move to the right.By appropriately changing the energization of the teeth, oscillation isproduced.

Similary, the in-plane actuator 40' has a set of teeth 90 on thepick-off plate 22' projecting parallel to the direction of desireddisplacement of the plate. A corresponding set of fixed teeth 91 ismounted on the wafer 1' and is interdigitated with the teeth 90. Byapplying different voltages to the two sets of teeth 90 and 91, theplate 22' can be pulled in its plane towards the actuator 40'.

In practice, the sensor will usually have a greater number ofinterdigitated teeth or fingers than described.

The sensors of the present invention could be used in a system formeasuring acceleration rather than rate.

The sensors of the present invention can be made at low cost bymicroengineering techniques and are susceptible to automatedmanufacture, trimming and testing. The sensors are very compact and canhave a high sensitivity.

What is claimed is:
 1. A rate sensor including a resiliently-mountedelement a first actuator that vibrates the element in a first plane (x),a displacement sensor responsive to displacement of the vibratingelement in a sensing direction (z) normal to the first plane caused byrotation of the sensor about an axis (y) in the plane and at rightangles to the sensing direction, a tunnel pick-off having a first partlocated on the vibrating element and a second part separate from thevibrating element, one part including a spike and the other partincluding a surface closely spaced from the tip of the spike, the sensorincluding a second actuator so that the first and second parts can bedisplaced relative to one another from a position prior to use in whichthe spike is protected.
 2. A rate sensor according to claim 1, whereinthe surface has a recess positioned above the spike prior to use.
 3. Arate sensor according to claim 1 or 2, wherein the spike is mounted onthe vibrating element and the surface is on the second part.
 4. A ratesensor according to claim 1 wherein the tunnel pick-off includes twospikes, the surface has a recess located above one of the spikes, andthe output of the spike located beneath the recess is utilized toprovide an output representative of vibration of the element.
 5. A ratesensor according to claim 1 wherein the sensor includes a third actuatorfor maintaining a constant average separation between the first part andthe second part of the pick-off.
 6. A rate sensor according to claim 1wherein of the preceding claims, the vibrating element is a platemachined from a silicon substrate, and the substrate supports the secondpart of the tunnel pick-off.
 7. A rate sensor according to claim 1wherein the first actuator that vibrates the element in the first plane(x) is an electrostatic actuator.
 8. A rate sensor including aresiliently-mounted element, an actuator that vibrates the element in afirst plane (x), a displacement sensor responsive to displacement of thevibrating element in a sensing direction (z) normal to the first planecaused by rotation of the sensor about an axis (y) in the plane and atright angles to the sensing direction, a tunnel pick-off having a firstpart located on the vibrating element and a second part separate fromthe vibrating element, one part including a spike and the other partincluding a surface closely spaced from the tip of the spike, thevibrating element having a flexure arm, and the vibrating element havingbeen tuned by removal of material from a surface of the arm.
 9. Atwo-axis inertial rate sensor system including four pairs of ratesensors according to claim 1 wherein the rate sensors in each pair (Y1and Y2, Y3 and Y4, X1 and X2, and X3 and X4) are mirror images of oneanother.